Electric transformer e



I. BIBBY.

ELECTRIC TRANSFORMER ESPECIALLY SUITABLE FOR USE IN CONNECTION WITHELECTRIC FURNACES.

APPLICATION FILED DEC. II 19H.

1,315,359." PatentedSept. 9,1919.

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UNITED STATES PATENT OFFICE.

JAMES BIBBY, OF LONDON, ENGLAND.

ELECTRIC TRANSFORMER ESPECIALLY SUITABLE FOR USE IN CONNECTION WITHELECTRIC FURNACES.

Specification of Letters Patent.

Patented Sept. 9,1919.

Application filed December 11, 1917. Serial No. 206,622.

To all whom it may concern:

Be it known that I, JAMES BIBBY, a subject of the King of Great Britainand Ircland, residing at London, England, have in vented Improvements inor Relating to Electric Transformers Especially Suitable for Use inConnection with Electric Furnaces, of which the following is aspecification.

With electric furnaces of the arc heating type it is necessary, with thelarger sizes thereof, in order to distribute the source of heat over thebath of metal or other charge therein (hereinafter referred to as metal)and, at the same time to keep the electrodes within reasonabledimensions, to use four upper movable electrodes. These four electrodesare arranged generally vertically over the bath of metal and throughthem equal amounts of current flow to form arcs between their ends andthe surface of the metal when the distance between the ends of theelectrodes and the metal are equal, so that the resistances are equal.Consequently, it is necessary, when the source of electric supply isthree-phase alternating current, to transform the three-phases into fourequal phases in such a manner that when equal currents are being takenby the four arcs, equal currents will be taken from the three primarymains.

It is also important for metallurgical reasons that the four secondarycurrents should not be in equilibrium, that is to say, that the vectorsum of the four currents should not be zero but that there should be abalance of current available for a common return fifth conductor andthat this return current should, generally, to obtain the best results,be at least equal to that in one of the four phases, but may be less,though with reduced effect. This return current flows through a neutralfixed electrode in the hearth of the furnace so that the out of balancecurrent, will flow in a more or less vertical direction through the bathof metal. The currents be tween the phases pass in a horizontaldirection. hen the vertical current equals or exceeds that in one of thefour phases, metallurgical requirements will, be satisfied.

Therefore to obtain the best results, a four phase current is requiredall the phases of which shall be equal to one another with respect tothe neutral, and also a return neutral current that shall exceed or atleast be equal to that of one of the phases, so that there is obtained afour phase five wire system in which the conductors can be of similarsectional dimensions.

A four-phase current can be obtained from a threephase current by a dualsystem of Scott connections but this requires the equivalent of fourtransformers with six sets of connections on the high tension side, or,if only two transformers are used, the four phases are simply two setsof two phases in parallel, which system has disadvantages.

The present invention has for its object to enable four equal phases to"be obtained from three single phase transformers, or from onethreephase transformer, in such a manner that the system shall bebalanced to a permissible extent, on the three-phase primary side whenequal currents are being taken in each of the four secondary phases andthe before-mentioned common return current is provided for; and also,that when the four phases become unequal through deficiency ofelectroderegulation, the currents in the three primary phases shallremain permissibly balanced.

For this purpose, in three phase transforming plant according to thepresent invention, the transformer ratio of two of the three singlephase transformers, or of two of the associated sets of primary andsecondary windings of the single three phase transformer, may be madeequal to one another and the transformer ratio of the third transformeror of the third set of associated primary and secondary windings of thesingle transformer, be made of the order of about 20% greater value thanthat of the other two. If therefore the three primary windings of thethree transformers or of the single three phase transformer areconsidered for instance equal to one another in value, then of the threesecondary windings of the three transformers, or of the singletransformer, two of them are made equal to one another in value, andeach of them is connected at its ends separately by conductore to two ofthe four electrodes and the third secondary winding, under thiscondition of equal primaries is made of the order of about one-sixthless in value than the other two, and is connected at each of its endsby a conductor to an intermediate point of one of the other twosecondary windings and at its central point is provided with a tappingfor connection through a fifth or a return neutral conductor to thefifth electrode arranged in the hearth of the furnace. If on the otherhand, the number of turns in allthe secondary windings are consideredequal in value to one another, then two of the primary windings will bemade equal to one another and the third primary winding will be made ofthe order of about 20% greater in value than either of the other two, soas to Satisfy the hereinbefore stated transformer ratio proportions. Thenumber of turns in the respective windings and the points in'the twoequal secondary windings to which the ends of the third and it may beunequal winding are connected, are such as to enable the above describedrequirements to be obtained.

The primary windings in each case can be connected together eitheraccording to the star or delta method of connection.

In the accompanying illustrative drawings Figures 1 and 2 show,diagrammatically, transformer arrangements suitable for obtaining from athree phase transformer plant, four equal phases for use with fourelectrodes and a balance current for a return conductor and fifthelectrode as hereinbefore described. These figures show the windings ofeither a single three phase transformer or three single phasetransformers. Fig. 3 is a diagram hereinafter more particularly referredto.

Referring to Figs. 1 and 2, the three primary windings A0, B0 and CO areconnected together at the point 0 according to the star method ofconnection and are, in this example, similar in value to one another,having the same number of coils or turns. VT, ZY and X are the threesecondary windings, two of which, namely X and ZY are equal in value toone another, having the same number of coils or turns, while the other,namely VT, is of different and less value than the other two, havingfewer coils or turns. Thus, wind ings X and ZY should each haveapproximately 20% more coils or turns than the winding VT, that is tosay, when the primary windings are equal to one another, the proportionof coils or turns in the windings lVX and ZY to the coils or turns ofwinding VT may be about as G to 5 so that the transformer ratio of thewindings -AO, VT is about 20% greater value than that of the other twosets of windings BO, ZY and CO, X. The opposite ends of the winding lVXare adapted to be connected by conductors a and Z) to the twoelectrodes 1. and 2 respectively and the opposite ends of the winding ZYare connected by conductors c and (Z to the electrodes f and 3respectively. The ends of the third secondary winding VT are connectedto tappings at intermediate points L and if respectively on the othertwo secondary windings \VX and ZY. These intermediate points L and K aresuch as to divide the coilsin these secondary windings into two portionsin the proportions of the order of 7 to 17 as shown. At the middle pointM of the third secondary winding VT is a tapping for connection by aconductor 0 to the neutral fixed electrode N in the hearth of a furnace,this conductor forn'ling the common return neutral wire of the secondarysystem. When the windings are thus proportioned, arranged and connectedup and the E. M. F.s and currents indicated by the references E and Irespectively in Fig. 2, in the primary windings are balanced, the E. M.F.s between the electrodes 1, 2, 5 and 4 and the neutral elect-rode Nwill also be equal, as indicated by the references E so that when theresistances between the four electrodes 1, 2, 3, and at and the fifth orneutral electrode N are equal, the currents which will flow between eachof the four electrodes and the neutral electrode, will be equal, asindicated by the references I and the required amount of current willflow back through the return conductor a between the neutral electrode Nand the middle point M of the winding VT to satisfy the before-mentionedmetallurgical requirements.

In the case where the secondary windings are made of equal value and theprimary windings are made of unequal value, then the desired transformerratio of one set of the windings to the other two sets should be of theorder of six to five. Thus with an arrangement such as shown in Figs. 1and 2 but under the modified conditions just mentioned the number ofcoils or turns in the primary winding AO would be 20% greater than inthe windings B0 and CO.

It can be readily shown that the herein before described arrangement oftransformer windings and electrodes fulfil all the required conditionshereinbefore mentioned.

For this purpose reference may be had to 1 and 2 and to Fig. 3, which isa clock diagram, and shows the system of connew tions when the starmethod is adopted for the primary windings. The arrangement is such thatthe transformer ratio of the set of windings AO VT is 20% greater thanthe transformer ratios of the sets of windings COiVX and B( )-ZY whichare equal. In practice these transformer ratios may be obtained in anyconvenient manner. For example, if the secondary windings are all equal,then the number of turns or coils in the primary winding A0 would ashereinbefore stated be about 20% greater than those in the primarywindings B0 and (K7 but by way of example, it will be assumed that thetransformer ratios are rcprcncni-ml by the number of turns or coils inthe windings on the secondary side, the primary windings being assumedto be all equal. Under this assumption, the turns or coils of each ofthe secondary windingsZ Y and X should, as nearly as convenient, bedivided at the point L and K into two portions in the proportion ofabout 7 to 17 as shown in Figs. 1 and 2, although in practice, for convenien'ce of manufacture, these intermediate points may be otherwisedistributed and rectifications made in the other windings, so as toeffect the same results.

Now taking the figures given on the diagram Figs. 1 and 2, the-pressuredifference between electrode 1 and the neutral 'elec trode N is equal toWK, plus MT reversed. In the vector diagram Fig. 3,.this can herepresented by the lines ()0 and Del giving the resultant '01. Similarlythe pressure difference between electrode 2 and the neutral electrode Ncan be represented by KX reversed, pluS MT reversed. These arerepresented by the lines 00 and 0d in Fig. '3, and give the resultai'it02-. Similarly we find the pressure difference between electrodes 3 and4 and the neutral electrode N as the resultants O3 and 0 1 respectively.It will be seen that the system induces equal pressures between each ofthe electrodes and the neutral.

The lines '01, O2, ()3 and 04 can be taken as representing thedirections of the currents flowing in the four electrodes 1, 2, '3 and4, so that if along each of these lines distances are marked off equalto the number of coils under their influence, the number of ampereturnsin each section of the winding can be obtained. Assuming that unitcurrent is flowing through each of the electrodes 1, 2, 3 and 41, thenalong line 01 a distance 09 can be marked 'oif equal to 7 and along line()2 a distance 0 can be marked -off equal to 17. The vector differencefg of those two lines will give the ampere-turns in the primary windingCO. Similarly the ampereturns in the primary winding BO can be found tobe equal to Me.

Now the current in the secondary winding VT of the third transformerA'OVT is the vector difference between the vectorsums of the currents inthe secondary windings of the other two transformers. The currentflowing from the point T to the point K is equal to the vector sum ofthe currents flowing to electrodes 1 and 2 and the current flowing fromthe point V to the point L is equal to the vector sum of the currentflowing to electrodes 3 and a. The difference between these vector vsumsis equal to the current flowing in VT and after taking into account thenumber of windings, the line hp is obtained as representing theampereturns in the primary winding A0. It will be seen that the linesf9. his and ma are all equal, and as it has been assumed for clearn'essthat the primary windings have all the same number of turns, then thecurrents in the prlmary windings are all equal. Fur* thermore, by takingthe vector sum of all the currents flowing through the electrodes 1, 2,3 and 4, the line ON is obtained as representing a current which wouldflow through the neutral electrode N assuming that the length of theline Ocl represents the normal current in each electrode 1, 2, 3 and 4tand that the health is a good conductor.

As the lines 01, O2, ()3 and 04 (Fig. 3) represent the respectivedirections of the currents passing through the electrodes and takinginto account the number of windings under the influence of each currentand the respective directions and magnitude of the currents, it will befound that when equal currents are taken through the arcs between theelectrodes 1, 2, 3 and t and the metal in the furnace then equalcurrents are drawn from the three primary mains, and also that thesecurrents are substantially in phase with their respective voltages, sothat the power factor on a non-inductive load is high. Furthermore, whenequal currents pass through the electrodes there is a balance or returncurrent which is equal to 'or greater in amount than that in one of thephases.

Again, Fig. 3 shows that if the furnace bottom does not for any reasonconduct the current to the return electrode N, then the currents in theprimary mains will remain well balanced, and have on a non-inductiveload, a power factor of practically unity.

Thus it will be seen from Fig. 3 that the hereinbefore describedarrangement of transformer windings and electrodes when combined with anelectric furnace enable the following features to be obtained, namely 1.A four-phase current with a neutral return current in a direction atright angles to the directions of the current between the phases in thebath of metal, so as to satisfy metallurgical requirements.

2. That when equal E. M. F.s are impressed by the primary windings,equal E. M. F.s are induced between each of the four secondary windingsand the neutral electrodes.

v 3. That when equal currents are taken through the four secondarywindings to the electrode as well as a suitable current to the neutralreturn, equal currents are taken from the three primary windings.

4-. That when unequal currents are flowing through the four secondarywindings, due to defective electrode regulations, or resistance of thehearth, the three primary currents remain balanced to a permissibledegree.

' 5. That the power factor of the system is high, sinre angulardisplacement of the E. M. F.s and corresponding currents in the primarywindings and mains is small under working conditions.

It is to be understood that the above mentioned proportions of the threetransformer ratios and the positions of the intermediate tapping pointscan be slightly modified without affecting the scope of the inven tionwhich includes such modifications.

Each of the electrodes 1, 2, 3 and 4: and the neutral electrode Ninstead of consisting of a single electrode may be replaced by a groupof two or more electrodes spaced apart.

\Vhat I claim is 1. A three phase transformer arrangement comprisingthree sets of primary and secondary windings, two of the sets of suchwindings being of the same transformer ratio and the other or third setbeing of dif ferent transformer ratio to the other two sets, thesecondary system of the two sets of equal transformer ratio having fourterminals and the secondary system of the third set being interconnectedwith the secondary systems of the other two sets, for the purposespecified.

2. A three phase transformer arrangement comprising three sets ofprimary and secondary windings, two of the sets of such windings beingof the same transformer ratio and the other or third set being ofgreater transformer ratio to the other two sets, the secondary system ofthe two sets of equal transformer ratio having four terminals and thesecondary system of the third set being interconnected with thesecondary systems of the other two sets, for the pur pose specified.

3. A three phase transformer arrangement comprising three sets ofprimary and secondary windings, two of the sets of such windings beingof the same transformer ratio an d the other or third set being ofdifferent transforn'ier ratio to the other two sets, the secondarysystem of the two sets of equal. transformer ratio having four terminalsand the secondary system of the third set being interconnected with thesecondary systems of the other two sets and connected to a fifthterminal for the purpose specified.

1-. A three phase transformer arrangement comprising three primarywindings and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings be ingequal "and the transformer ratio of the third pair of such windingsdifferent there to, terminal connections from each end of the secondarywindings of the two pairs of windings of equal transformer ratio,connections from the ends of the third secondary winding to pointsintermediate of the remaining secondary windings and a terminalconnection intermediate of the third secondary winding, for the purposespecified.

5. A three phase transformer arrangement comprising three primarywindings and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of the third pair of such windings ofgreater value than that of the two pairs of equal transformer ratio,terminal connections from each end of the secondary windings of the twopairs of windings of equal transforuu-r ra tio, connections from theends of the third secondary winding to points ii'itermediate of theremaining secondary windings and a terminal connection intermediate ofthe third secondary winding, for the purpose specified.

6. A three phase transformer arrangement comprising three primarywindings and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of the third pair of such. windings ofgreater value than that of the two pairs of equal transformer ratio,terminal connections from each end of the secondary windings of the twopairs of windings of equal trans former ratio, connections from the endsof the third secondary winding to points intermediate of the remainingsecondary windings and a terminal. Connection at the central point ofthe third secondary winding, for the purpose specified.

7. A three phase transformer arrangement con'iprising three primarywindings and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of the third pair of such windings ofgreater value than that of the two pairs of equal transformer ratio,terminal couiicctions from each end of the secondary windings of the twopairs of windings of equal transformer ratio, connections from the endsof the third secondary winding to points of the remaining secondarywindings dividing the latter each into two parts of unequal.proportions, and a terminal connection intermediate of the thirdsecondary windimr, for the purpose specified.

8. A three phase transformer arrangement comprising three primary winjlinga and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of. the third pair of such windings ofgreater value than that of the two pairs of equal transformer ratio,terminal connections from each end of the secondary windings of the twopairs of windings of equal. transformer ratio, connections from the endsof the third secondary winding to points of the remaining secondarywindings dividing the latter each into two parts of unequal proportionsand a terminal connection at the central part of the third secondarywinding, for the purpose specified.

9. A three phase transformer arrangement comprising three primarywindings and three secondary windings magneticallyassociated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of the third pair of such windings beingof about 20% greater value than that of the other two pairs of equaltransformer ratio, terminal connections from each end of the secondarywindings of the two pairs of windings of equal transformer ratio,connections from the ends of the third secondary winding to pointsintermediate of the remaining secondary windings and a termi nalconnection intermediate of the third secondary winding, for the purposespecified.

10. A three phase transformer arrangement comprising three primarywindings equal in value to one another, a pair of secondary windingsalso equal in value to one another and a third secondary winding oflesser value, terminal connections from each end of the pairs ofsecondary windings of equal value, connections between the ends of thesecondary winding of lesser value and points intermediate of the othersecondary windings and a terminal connection intermediate of the saidsecondary winding of lesser value, for the purpose specified.

11. A three phase transformer arrangement comprising three primarywindings and three secondary windings magnetically associated therewith,the transformer ratio of two such pairs of associated windings beingequal and the transformer ratio of the third pair of such windings ofabout 20% greater value than that of the two pairs of equal transformerratio, terminal connections from each end of the secondary windings ofthe two pairs of windings of equal transformer ratio, connections fromthe ends of the third secondary winding to points of the remainingsecondary windings dividing the latter each into two parts of unequalproportions and a terminal connection at the central part of the thirdsecondary winding for the purpose set forth.

12. A three phase transformer arrangement comprising three primarywindings equal in value to one another, a pair of secondary windingsalso equal in value to one another and a third secondary winding oflesser value, the proportions of turns in the primary winding associatedwith the latter winding to the number of turns in such secondary windingbeing in the proportions of about six to five, terminal connections fromeach end of the pair of secondary windings of equal value, connectionsbetween the ends of the secondary winding of lesser value and pointsintermediate of the other secondary windings, each point dividing eachof such other secondary windings into two parts in the proportion ofabout seven to seventeen and a terminal connection at the central partof the third secondary winding for the purpose set forth.

Signed at London, England, this eighth day of November, 1917.

JAMES BIBBY. lVitnesses:

L. M. GLADDIs, W. SENSLOS.

Copies of this patent may be obtained for five cents each, by addressingthe Commissioner of Patents, Washington, D. G.

